Fractures Characterization And Controlling Effect On Shale Gas Of Lower Cambrian Niutitang Shale In Cen'gong Block, Northeastern Guizhou Province

Marine shale in southern China has many unfavorable factors such as old age of formation,complex tectonic deformation and poor preservation conditions.At this stage,relatively fewer studies have been conducted on the developmental characteristics and activity histories of shale fractures,the specific role of natural fractures in shale gas enrichment and conservation is unclear.Based on field surveys,drilling cores,interpretation of 2-D seismic data and analysis of a large number of experimental tests,the paper finely characterizes the developmental characteristics of fractures at different scales,searches for fluid geochemistry evidence of fracture activity,accurately delineates the developmental period of fractures,explores the history of fracture activity,and summarizes the role of reservoir control and shale gas control of fractures in marine shale of complex tectonic zones in southern China.The results show that the Cen'gong block is a saddle-shaped structure supported by the northeast oriented Banxi anticline and the south-west oriented Lanigan anticline.Compression and strike-slip two tectonic patterns are mainly developed.Fractures in Niutitang shale are mainly divided into outcropping fractures,core fractures,microscopic fractures and logging fractures,in which the core fractures are mainly developed as horizontal pyrite-filled fractures,bedding fibrous veins,vertical filled fractures and slip fractures.What's more,the vertical filled fracture developed at least two periods.The intersecting relationships of the core fractures show that the horizontal pyrite fracture formed earliest,followed by bedding fibrous vein,then vertical filled fracture,and finally a slip fracture.Both the field fracture rose diagram and the paleomagnetic orientation experiments show that the fracture direction is mainly NNE and partly NE,which are basically consistent with faults direction interpreted by the seismic data.By summarizing the developmental characteristics of field fractures and core fractures in the Niutitang shale in the study area,the paper summarizes four fracture development patterns: intra-layer confined,inter-layer transitional,layer active and composite.Fracture filling minerals in study area are mainly calcite,quartz,barite,dolomite,barytocalcite and norsethite,followed by small amounts of iron dolomite,siderite,rhodochrosite and other related minerals.Comparing the features of the fracture-filled minerals by common optical,cathodoluminescence and backscatter scanning microscopy,it is evident that the fracture opens and closes in multiple phases,and the stable carbon and oxygen isotopes of the filled minerals further indicate that the fracture has a six-period filling process.The filled minerals developed pure liquid and gas-liquid aqueous inclusions and pure liquid phase methane inclusions.The homogenization temperature of aqueous inclusions can be clearly divided into three low,medium and high temperature intervals,following three low,medium and high salinity intervals respectively.The methane inclusions density was calculated using two methods,the uniform temperature parametric method and the laser Raman displacement parametric method,which is much larger than the critical methane density value.The trapping pressure was calculated to be 59.41~90.80 MPa,and the stratigraphic pressure coefficient of the Niutitang shale was 1.22~1.86,which means the stratigraphy was in a state of overpressure.The porosity of Niutitang shale in the study area increases with the density of the fracture body,indicating that the fracture has a significant effect on the development of the pore of the shale reservoir to a certain extent.Secondly,the greater the fracture line density,the greater the shale reservoir brittleness index.The hydraulic fractures in the TX-1 well are in good agreement with the natural fracture developmental extent and extension direction,both of which extend primarily along the NE direction.The higher the natural fracture development,the lower the reservoir bursting pressure can be seen by comparing the bursting pressure and fracture development of TX-1 and CY-1 wells.The lateral comparison of TX-1,CY-1 and TX-1shows that the drilling well is progressively closer to the fault,the fracture development level is progressively larger,and the gas content is progressively worse.The longer the interval between fracture formation and mineral filled,the less conducive it is to shale gas preservation and enrichment.There are four types of fractures and six stages of development in the study area.Different stages and types of fractures have different effects on shale gas enrichment,specifically,Stage I horizontal pyrite-filled fractures,Stage II bedding fibrous veins and Stage III vertical filled fractures are conducive to increasing shale gas storage space and improving reservoir seepage capacity,while Stage IV vertical filled fractures,Stage V vertical filled fractures and Stage VI slip fractures have damaged shale gas storage conditions and caused shale gas dissipation.The research results of the paper will provide reliable basic information for the next steps of shale gas well site selection and design,horizontal well drilling and hydraulic fracturing in the Cen'gong Block,and also provide scientific basis and theoretical guidance for the exploration and development of marine shale gas in the complex tectonic zone in southern China.